But apparently not before the tweet caused a dramatic drop in the market, as Barron's reported. The market rebounded after it became clear that the "news" was actually a hack. But as Matt Yglesias argues in Moneybox, that dramatic effect could bode well for some opportunists:

"It makes you wonder if there isn't a profit-making opportunity out there in the realm of Twitter hacks. The Internet seems to spread both the rumor and the correction very quickly, and U.S. financial markets are deep and liquid enough to respond in a big way in real time."

The AP's Mike Baker tweeted out more information on the hack, noting that at least one other AP account had been taken over. From what the AP is saying, it looks like the hack was actually the result of a phishing scheme that was emailed to some AP employees just before the accounts were compromised: